Try making new app pool
Try making new site+app pool
Do other sites work?
If only SedonaWeb, does turning Enable 32-bit applications to "False" get a .Net application error screen instead of error 503 or similar?
If so:
Check Windows logs. Might give the specific .dlls that are likely the culprit.
Known .dlls that are 64-bit: Exchange-related .dlls, RPC host dll
Go to Windows/System32/inetsrv and make a backup of applicationhost.config
Go into the applicationhost.config file and find the global modules (should see lots of references to .dlls)
Add at the end of each 64-bit dll's line before the closing />: preCondition="bitness64"
