Does it look kind of grainy and/or separated, along with being watery? If so, your gravy has "broken' (the fat in the sauce has separated from the emulsion). This is incredibly common and can happen if the gravy is re-heated too hot, and sometimes occurs simply by refrigerating it, depending on how strong your emulsion was in the first place (extreme temps will pull the fat out of the emulsion).
I'd tend to say just toss it when it gets to the point of being completely watery (I don't feel like it recovers to it's original best, no matter what you do), but you can do a decent job of fixing it by making a roux (equal parts fat and flour, whisked together in a pan on med-low heat) then turn up the heat to just above medium and whisk in your broken gravy, just a tiny bit at a time, whisking vigorously after each addition. You should end up with a smooth and thicker, re-emulsified gravy at the end. Good luck!
If your gravy hasn't broken yet and you're just trying to prevent it, then be very careful not to re-heat it too hot. Microwaving it on high power will kill it, for sure! I'd reheat it on the stove top at a reasonably low temp, whisking often. It should hold together just fine for you that way.
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