Don't despair! It is not going to be that bad at all...........
The first challenge it to take the plant(s) out of the pot and as you have suggested - take a look at the roots. I would not expect any bugs or creepy crawlies to come after you.

Any roots that are soft, black, dried up looking, etc. can be removed. Only the tan/white/green plump ones should remain. If you have not got much left it could call for drastic measures beyond just repotting. Leaves yellowing and falling off may not be abnormal if it is the bottom ones - they usually loose one or two every year and grow new ones to replace them. Can you take a digital picture of the depotted plant with remaining roots after removing the bad ones? That might give us some idea of what you are up against. If there are not many roots left you will need to give it some TLC. If we can look at a pic we can give you an idea of what to expect in terms of how long it is going to take it to return to healthy blooming condition and then you can decide if you think it is worth the effort or not. A signicantly declining plant can take 1-2 years to revive. Good luck!