My Bridge Fell Over! What Do I Do?
Oh no! Sorry to hear that your bridge fell over. It's usually a pretty scary thing, if you witnessed it happening, as it's accompanied by a LOUD bang/crash, and often makes you worry about the possibility that anything got seriously broken. The good news is that it is pretty easy to replace, provided that the bridge is okay. So we'll look at the bridge first.
Damage to the bridge is usually readily apparent. If it broke, you should be able to tell - you've got (at least) two pieces where before there was only one. Or, if it's an adjustable bridge, one or both of the adjusters may not operate or stay put in the bridge anymore. So, if the bridge is broken, you'll likely need a new one - but if not, let's soldier on.
Look at the bridge and ensure that it has not warped over time; if it was properly fitted, one side should be totally flat/straight, the other side will likely have been thinned and shaped into a rounded off sort of profile. The bridge feet should be perfectly shaped to match the unique shape of the wood top it sits on so that it doesn’t lean, rock, or otherwise seem unstable. The top arch is probably lower on one side compared to the other, and that lower side is probably the "G" side of the bridge as the "E" string usually needs a bit more clearance to vibrate.
Once we've figured that out, let's check to make sure that the soundpost didn't drop out of place when the bridge fell over. If you use a flashlight to look into the f-hole on the "G" side of the bass, you should see a wooden "dowel" that spans from the front to the back of the bass, roughly close to where the G-side bridge foot "lands" on the bass top. If the soundpost is not in place, DO NOT reinstall the bridge - that soundpost carries the vibrations from the front to the back, but it also acts as crucial structural support for the tension that the bridge bears on the top. If you reinstall and tune up without that structure in place, the bass top can deform/cave in/crack/etc.
Assuming that the bridge is fine and the soundpost is in place, let's get the bridge back in place.
Loosen the strings enough to put the bridge in place under the strings, and the strings can gently hold it in place as you carefully tune up. The two f-holes should have “notches” somewhere near the middle. If you draw an imaginary line between the two inner notches, that’s the line on which the bridge should sit, centered on the line and the front of the bass. As you put tension on the strings and tune them up, monitor the bridge. If it starts leaning towards the neck (common), tap it back fully upright so that it’s perpendicular to the body.
This is something you should regularly monitor, moving forward – the bridge, as you tune it up to play on any given day, may get slightly “pulled” towards the neck by the friction and tightening of the strings. Straightening it back up will prevent another “fall down” as well as help to prevent possible warpage (which can occur because the tension is stressing the bridge unevenly if it’s not standing straight up.)
Once you get back up to tune, you should be good. Keep an eye on the bridge to make sure it stays straight up and down, and listen carefully for any new buzzes or rattles that may have cropped up when the bridge fell. If you have any, you may want to have the instrument looked at for hidden damage concerns. If the bridge does not stay straight up and down, or doesn't sit square on the top, you may need to refit it (or have someone else do that for you). It should be quite stable, and if it's not, it will probably fall down again.
Hopefully that gets you back up and running! Let me know if you have any further questions.