To reinstall you will need
1. The installer. This might be the original CD/DVD disk, or the original download. (Always be sure to archive downloads OFF YOUR COMPUTER).
2. The original serial number.
3. If it was an upgrade, the PREVIOUS ("qualifying") serial number.
If you get a message about too many activations, you will need to contact Adobe. This is routine, but having proof of purchase might be helpful; allow plenty of time for the job.