I have just 1 beam that is no longer in the notch that is supposed to rest in on the crossbeam. The house is 100 years old. Would it be a safe option to install a floor jack to bring the beam back up to where it had been? The floor is bouncy due to this. The joist is in an area where there is a lot of foot traffic.
As stated in everybody's comments, this is not a place to go blindly.
For the joist to "slip out of" its notch, something else has probably moved. This can lead to a house-sized game of "Jenga®" where the players (being you and anybody else in the house) can be crushed by the falling timbers.
Hire yourself a civil/structural consulting engineer, preferably one that really understands old wooden houses if yours is (as it sounds like it might be) a post and beam (P&B) structure. Those working for new P&B companies might be your best bet, and if they are not, they will usually know someone who does do work with old P&B houses, in most cases.