My thorens td-190-2 recently lost its normal output volume. I have tried three cartridges with no change. What could it be?

I assume your turntable was connected to the record deck input on your amp when you lost volume from it. Then you tried other connections marked tape or CD?
As you have tried different cartridges with no response we can assume the piece of equipment that has failed is not your record deck. Instead it is your amp. The problem you are experiencing is that the pre-amp inside your amp has failed. This pre-amp is connected only to the record deck input of the amp. The other connections don't have this pre-amp which is called a magnetic cartridge pre-amp. So connecting to them will just give you the signal only from the deck, which as you say you can hear, thus proving the cartridge is working, but will be low in volume.

If you are any good at electronic repair it will be easy to fix this pre-amp. You can find it by following the internal connections from the record deck socket back to a circuit board. The chances are this will lead to an IC that does the work. Get the IC number and search the web for it, before removing it.
Alternative you can take it to a service centre for repair if you have no experience of electronic repair.
Another option is to search the web for a magnetic cartridge pre-amp. These are sold separately as many Audio amps don't have them fitted, seeing not many people have record decks. This can be connected between the deck and one of those other inputs (don't use the record deck one). The cost of one of these might work out cheaper than repair at a service centre. The repair by yourself will of course be cheaper, unless the IC is hard to find or specialised. But a search of the web for the IC number will show how cheap it is anyway. However if the IC is expensive that will also make repair by a service centre expensive too. So you can save the expense of the repair by going for a new pre-amp.