My cars been making a high pitch squeal which I hear could be the fan, the alternator, or the belt (which was worked on last). I didn’t feel like fixing it, because it didn’t seem to affect how I drove, but now when I come to a stop, and push on the gas it has no power, and it seems (I’m not sure if I’m imagining this) to happen again around 20mph - when it would shift. It just happened this morning and I haven’t drove it after it warmed up a bit.
If the car has air conditioning, the compressor could be seized up. That would be the cheap problem. More likely your transmission is giving out. That will be much more expensive to fix.
In any case, I would take it to a good mechanic to determine what’s actually wrong. If it the transmission you may have to scrap the car since any fix will be much more than what the car is worth.