I have RCA L42WD22 TV comes on then turns it's self off. I have followed the manual to reset TV but that doesn't work.

This sounds to me like it is a fairly classic switchmode power supply problem, either with the PSU, and/or some high current section is failed, the protection circuitry built into the set, has sensed a severe potential, or actual existing, problem, and is shutting down to avoid a fire, explosion, or both. This set had developed a problem, that is so severe that the set must be shut down, This set should be removed from the Power, and in my professional recommendation, you should get a "Quote" for repair, from, an, Approved Service Center, this point is very important for a good result, Always contact, their local, "Head Office" Now, when you get the information you want, then you can decide as to repair of replacement. The reason we want an approved agent, is that way, all the engineers are trained on their own brand sets, they have seen almost all faults, and will get the job done quicker, and for less cost, and usually have parts on hand, also sometimes, there has been a service bulletin in issued and the repair may be subsided or even free. There are many, many, many reasons that display this symptom, one must work through the troubleshooting steps to isolate and identify, the Cause and repair that and any damage done.
http://www.bitmanuals.com/parts-111022154/RCA+L42WD22,L46WD22,etc.+TV+service+manual

Above is a service manual for your convenience.

With this symptom, it is impossible to tell what the exact "Cause" of this symptom is. These days unlike the "Old" analogue days it just isn't realistically possible to fix anything but the most basic of problems with todays sophisticated units. This is quite apart from the Safety aspect, any unit that can cause a fire or cause an explosion, or both, is NOT to be treated lightly, and and the repairer, of the unit is legally responsible, another thing to think about. Also, one must have tools, a Multimeter, a soldering iron, a deslodering iron, an oscilloscope, a signal tracer, tools, and most important of all, a Service Manual, without which, there is no way to even begin to understand and fix it, and more importantly Program it, AFTER it has been fixed. As these days a TV, is a PC, a Tuner, and an HDTV Monitor, and more, all in one. in the "old" days TV"s were more, rudimentary, and built to last for 20 years, and a "Parts" supply was set aside. They usually used to fail for only a few reasons, and they while still difficult, were less difficult to repair. Electrical knowledge was transferable, and they could be and would respond to, This-is-the-symptom-This-us-the-Fix, But that no longer applies, as now, it can be a Fault on I squared Bus, or "Sensor" failure, or it simple may have "Crashed" just like a PC, It could be a Diode go bad, and that will stop the whole thing. You see, in the day, the sets were Very "Fault Tolerant" because they were Analouge, and a faulty, component, or more sometimes, the set would always diplay something usually, and when they didn't it was one of only a few things, sure there are always the horrid ones, and there are pleanty of those, but all in all it was quite easy. But with Digital Electronics, it either goes, or it doesnt, there are usualy few half measures. I mean thats the definition of logic either going or not. So it can take very little to completely paralyse a set, and generally give the only sysmptom, it can, by not working, usually No picture, or the like. So you, see the list is endless almost for any given SYMPTOM, as that is what is being seen, not the cause, as I keep going on about. Also any fault, usually involves the Power Supply, it is either the cause, the cause of the damage, or it is damaged. or ALL of them. Then after you have done the electronic repairs, you then must program the set to work, and align, and set up in general, any one of those settings missed or done wrong, back to square one. So, it is nearly always my professional opinion that one should always get a "Quote" form the manufacturers recommended service center, I cannot stress how important that last point is. It is even worth the cost of shipping. you see an authorised service agent, will have trained staff, trained on their brand, also they have first hand access to parts, original parts. This will actually save you money and ensure you can actually get it back fixed. Of course asking for a "Quote" also gives on the option of repair or replace. Often my clients,do not go through with repair, unless it is moderate, but in some of the more costly jobs they see it, as do i, that a replacement, with a warranty, is a better option. As an aside here, one could then tear the TV down, and sell the Known Good bits. You'd be surprised who needs parts, i myself often must by from third party sources to effect a repair. So money could be recouped, somewhat,and help out someone else. Anyway thats my 10c worth.