Panic. Getting flustered, embarrassed and/or teary isn't going to get the cash together, nor is pretending everything is OK. You need to stay calm and work out a plan of action. Remember, you can't be evicted without a court order, and that could take weeks to arrive.
Don't ignore it.
Expect friends or flatmates to bail you out. If you're only a little behind, your mates may offer to help you out, but making a habit of this won't go down well.