Usually, this is caused by an admin changing the prefix.
Since Groovy will always respond to a mention as a prefix, you can simply use @Groovy prefix to check if the prefix has changed. If it has, then it can quickly be fixed by typing @Groovy prefix reset
If you were unable to tag the bot, then it may have been kicked or blocked from viewing the channel you're typing commands in. If you're an admin, you can quickly check whether Groovy is still in your server by opening the Members tab under the Server Settings and searching for "Groovy."