Never detach and then attach a content database that hosts project server web access site. It looses its link with the SSP that created it and users will get the below exception when they try to access the web access site,
401 Unauthorized access Exception.
"As best practice maintain all project workspace site collections as part of a different database than the pwa site collection." otherwise they also loose the link the web access.
Navigate to SSP which created the PWA
1. Take SQL backups of all PWA databases.
2. Create a new PWA
3. Provide the same name of the pwa that existed along with all the exact same names of databases that are already created.
4. Try to provision the PWA
5. Note that the account used to perform the above activities should be the SSP service account.
Once the PWA is provisioned the site should open fine.