Ever since the last update (March 27th, 2018), when you press ‘Exit Game’ while in a server, the entire game freezes up as well as your PC. You can’t alt+tab out of it either, only solution I’ve found is rebooting the entire system to clear the frozen FiveM client.
Managed to grab a screenshot of when it freezes -
This is a rather annoying bug to say the least.
Same for me 
I have to exit the process every time witch ALT + F4
Same here. It happens to me too
it happens to me too
Same. All updated Drivers, Cleared FiveM Cache, Reinstalled Nvidia Drivers, Rolled Back Drivers, Tried Windowed Borderless , Full Screen, and reinstalled FiveM. No Fix.
It used to happen to me. The best you can do is press Alt+F4 a bunch of times until it closes.
For people having this issue, once it freezes, press Windows + D, or Alt + Tab, then right click your task bar, select Task Manager, then end FiveM. 
It’s a bug. Bugs happen. Just chill and I’m sure they will fix it. In the mean time there’s a fix above or if you have more than one monitor, my case 3, then after alt 4 and it freezing you can click off and click back on and it will say it’s not responding and you can cancel it.
Resolved in the latest game update.