Computer has bestec atx-300-12e power supply. it

Replace the power supply or have it tested.
Amazon com bestec atx 300 12z You can get a replacement pretty cheap. Most all power supplies can also be tested - Bestbuys geek squad or a small computer repair shop should have the means to test it.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/william_02fc2766c1e13b73

Take the power supply to a retailer or computer shop in your local area. Pretty standard from what I have found out.
Bestec ATX 300 12Z Rev DDR Replacement Power Supply HP 5188 2625 Have a great day.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/william_02fc2766c1e13b73

If you didn't accidentally unplug the four pin connector near the CPU, you need to replace the power supply. This is actually a fairly common occurrence with the cheap ATX power supplies being tossed into systems these days.

The quickest and easiest method.
Beg or borrow another power and connect it to your computer.

Most computer testing facilities will test your current power supply for free. A replacement is very inexpensive to replace. Bestbuy, MicroCenter, small computer shops and buy replacemnent from Amazon.com, newegg.com.. Just make sure you have the correct information for your replacement. The model number on the power supply would be your best bet in most cases. Good luck and have a great day. I buy power supplies as low as $15 dollars Generic

If you need further help, I’m available over the phone at https://www.6ya.com/expert/william_02fc2766c1e13b73

Hi,
You can find the same psu at ebay with free shipping.

http://itemlistings.ebay.com/sdcsrp?fl=360131455504&product=bestec+atx-250-12e&sconstraints=PriceMin.Value%3D23.00%26PriceMax.Value%3D45.59

check on mother bord there is power on key in corner of motherbord.
Some smps has switch in back side of it.

How about the LC Power LC350M Micro ATX - 380 Watt

or the 

Logisys 350W Micro ATX

They should fit your form factor and output as much as 350W being relatively cheap, but neither gets 18 amps on 12V rail...
I think no power supply of that size has an output like that...think about a cheap but substancial chassis upgrade if you need that power...Have a look at directron for the PSUs.

Hipro- HP-P3527F3W
is the replacement for Bestec 300W

no the supplier is trying to make some extra money they alway try to say stuff to make extra green that will work fine