The LR44 is dead and needs to be replaced. You will need a #0 Phillps to change it on the TI-84. Once you remove the cover as the screw is captive, pop the old cell out with a flathead screwdriver and install a new one. Refer to this photo as to where it is replaced:
In addition to that photo, this guide also shows how to replace it. The Plus Silver edition is the same thing as the Plus with removable faceplates.
After changing the coin cell out, check the batteries and how long they were installed. The TI-84 is power hungry (relative to a basic scientific calculator) and will be more picky about battery voltage. If you can switch, using NiMh cells will be better for your wallet and calculator, since it removes the risk of a leak entirely - but if it happens you’ll need to clean it up like an alkaline leak and replace the batteries as a set to be safe. If you can find a high capacity set of 4 (more being ideal) (with 850mAh minimum/900+ preferred), ditch the Alkalines for good. You can get away with carrying 2 sets in most cases, but if possible see if you can bring a battery charger (anything will work, but this is what I use) with you so you can always have a charged set that gets circulated. If mine is too expensive, this model looks like a descent option - but it doesn’t handle NiCd or LiOn cells.
In the future, it’s also a good idea to note when the batteries and backup battery were replaced so you have a aging point, especially for the coin cell as these last ~5-10 years, unlike a set of alkaline batteries that die in ~6-8 months.