There has been a current upsurge in bans for hate-speech, judging from the threads being posted on the Boards. If you were reported for using any sort of racial or homophobic slurs (calling teammates n%%%%% or f%%/f%%%%%) it could result in an immediate escalation to a permanent suspension. It can take using the words more than once in a match, but Riot has a zero-tolerance policy towards hate speech, and people have been complaining about it recently. So, this seems to be Riot's way of letting players know that they are in fact listening.
Other than hate speech, players can also be immediately permabanned for any of the non-toxicity related rule violations. Things like botting or scripting (which are considered use of a malicious third party program), account sharing, or elo boosting can all result in an immediate permanent suspension when detected by Riot.
As the others have said, be sure to check your spam/junk email folders. Sometimes the emails are sent there by mistake. If you were issued the ban for toxicity, then trying to log in will show you the pop-up and a reform card which will have the logs of recent matches that contributed to your ban. As you have deleted the game, you may want to submit a ticket to Player Support. They will be able to tell you exactly why you were banned, and whether or not it was due to any sort of error. When/if you submit your ticket, it may take a little while for support to reply, depending on the volume of tickets they have at that time. Try to be patient, and do not submit a duplicate ticket, as that drops your request to the end of the line.
There are instances where a potential account compromise is detected, and Riot locks the account for the player's safety. In those cases, it will show upon trying to log in that the account has been suspended, and the player is encouraged to contact Support as soon as possible to resolve the issue. When this happens, it is usually due to an issue with chargebacks or a potential financial discrepancy.
Best of luck with your situation.