Before you upgrade your Orchard Website to Orchard 1.8, I recommend you read the Orchard 1.8 Release Notes. There are some steps formatted in BOLD that clearly state you must assign the Administrator Role to your Orchard Super User Account before performing the upgrade. If you don't, you will get locked out of the dashboard and have to either 1) re-do the 1.8 upgrade again after performing the step or 2) gain access to the Orchard database and do a bit of manual hacking.
Note that these instructions could change at anytime so I recommend you view the release notes directly, but here are the clearly stated instructions that need to happen prior to the Orchard 1.7.2 to Orchard 1.8 upgrade:
BEFORE DOING ANYTHING PLEASE FOLLOW THIS STEPS:
If you're confused about which account is the Super User Account, it is the account named in the General Settings area of the Orchard Dashboard.
Once you have identified the Super User Account, go into Users, edit the Super User Account, and assign the user to the Administrator Role.