If you used mods and have issues, just verifying the integrity of game files might not suffice, since Steam does only check if the files native to the game are okay. You would need to consult the mod(s) author(s) for further assistance. If you just want to get your game working again regardless whether you can continue to use your mods or not, use this guide
https://gtm.you1.cn/sharedfiles/filedetails/?id=2828650446

It is a standard procedure, addressing a very common issue. It happens even mostly automatically without you ever noticing it before, unless you paid close attention. When you look up this guide then it is mostly because the automation failed at some point, and you started noticing issues with running the game.

The common issue and reason behind the need is simple: games have large files. Whenever you copy stuff, or download in this case, there is a chance that something can go wrong. Countless sectors of your storage drive are being used in numbers I cannot nor want I to speak out loud without using the abbreviations of gigabytes.

The bigger the file sizes a game has to support, the more likely corruptions can happen. Especially with patches. Imagine it like a pair of jeans, patched up again and again. And at some point the stitches go wrong and the pair of jeans become unusable. You get two options then:

a) buy new jeans (aka redownload the whole game)

b) check the jeans you have and compare it with how it is supposed to look to fix the issue (aka verifying the integrity of game files), and then fix the parts which do not match the model

Normally when a patch is downloaded, Steam verify the files automatically and will attempt a redownload right away. But like services of your operating system, Steam services too can get stuck at some point. Having a hiccup and not working right - call it however you like. When the automation fails that is when you need to trigger this process of self-checking manually. Nothing more, nothing less to describe the whole process.

Speaking of "stuck services" - that is the reason why I urge you to reboot your PC after verifying the files. Even if there is nothing to redownload, we want to minimise the source of issues so you can enjoy your game as good as possible as soon as possible again.

For more tips and tricks in regards of troubleshooting, have a look here
https://gtm.you1.cn/sharedfiles/filedetails/?id=2834184614