I've dropped my phone a few times and now the S-pen isn't recognized.

I started out with a Samsung as my first smartphone, and it came with an S-Pen. What I learned that's relevant to you is that the phone actually has two digitizers; one for your finger and a separate one for the S-Pen. Having seen dropped phones knock out a standard digitizer, I see no reason why the same thing couldn't happen to the S-Pen digitizer.

If that's the case, then the fix will be that you'll have to replace the screen to get a new digitizer since it's all one assembly. Modern phones no longer make it possible to replace a digitizer separately from the display, as they're bonded together and require specialized tools, equipment and expertise to separate them and rebond the display to a new digitizer.

Unfortunately, iFixit doesn't have a guide to this repair yet, but I did locate them over at the Spanish site Nadie Me Llama Gallina. The guide is in Spanish, but if you use it with the Chrome browser's translation function, it is more than usable.

Manuals / Samsung Galaxy S23 Ultra / Full screen - Nobody Calls Me Chicken

Of course, the big problem is that the phone, being new, has not had time for the price to come down on parts, so I'm afraid you're going to pay a pretty penny for a new display. You can usually find the cheapest ones on AliExpress, followed by eBay and finally Amazon. Of course, a general internet search will probably turn up other vendors as well, but I'm going to have to leave that up to you. Here's one on Amazon that should work; be sure to check the model number of yours against the screen being sold; I did see one that said it was for the International version rather than the US domestic on, so be sure and check.

Amazon.com: 6.8" Original for Samsung Galaxy S23 Ultra SM-S918U SM-S918U1 SM- S918B SM-S918W SM-S918E LCD S23 Ultra Display Touch Screen Digitizer Assembly Replacment (with Black Frame) : Cell Phones & Accessories

That would be my best guess; of course you'll want to run any diagnostics you can find and possibly consider doing a full restore to factory settings just to be sure it's not a software issue, but if that doesn't help this is the most likely fix.

Note that most vendors will let you return the screen as long as it hasn't been installed, which means you could theoretically open up the phone and remove the battery so you can get to the display connector, then plug in the replacement screen to the motherboard so you can test whether it works or not before going through the entire process of replacement.

Good luck with it; be sure to come back and let us know how it all turns out and what you had to do to fix your phone; that way the next person with the same problem can benefit from your experience.