It depends on how you left the conversation. There are two options: 1. If you kept the default (accept future messages) then what happens is that the conversation is removed from your list of conversations and thus you'll no longer be able to open it and respond further. If, however, someone else in the conversation does respond, then it will be restored (brought back to normal). 2. It you selected the second option (ignore future messages) then, no, you will not be able to rejoin it under any circumstances. Another point is that for all the other members of that conversation, they will still see the same list of invited people. It doesn't matter which option you chose, you will still be listed as being part of the conversation. They will not know that you left.
