First take the following steps:
Uninstall the game from control panel's programs and features, or the apps list from whatever operation system you running.
Seek out the game's directory ("c:/riot games/league of legends" by default on windows), and if it exist, then DELETE it.
Download the game again, and install like if it were a new. Make sure, that the place you install doesn't exist yet.
Try it on custom game vs AI, if the issue still stands. So you don't ruin anyone's game, if yes.
Then try again with all known background program closed. Web browsers, steam, anti-virus program (disable, if you can't close it), etc.. The only program that should run, and be active is the League of Legends.
If the issue still stays, then check the forums, if anyone posted it. You might not be the first.
If yes, then send a reply to that post, and write what you have tried so far, and details of your system.
Operation system's version: Such as Windows 10 Pro v1803
Event connected to the crash in Event Viewer, or whatever log your system uses, if you can find it.
If there is a game crash log, then send it, and also post it on forum. Who knows. Someone might actually give an answer about it.
Also tell us, if you played the game since the last operation system (re)install without issue. It might be that one or more requirements are not (re)installed yet.
If nobody posted it, then create new post, and give all the details i mentioned above.