Did you follow the instructions about updating yum before updating the entire OS in the warning box at
http://wiki.contribs.org/SME_Server:Documentation:FAQ#Yum_Updates?(If not, you may have an incorrect kernel installed from the Centos repositories).
If you started with SME 7.0, you may also need to enable the default SME yum repositories (see the above link again) and run updates again.
Have you loaded any contribs? There was a post recently where server-manager broke after update -- I think that issue was related to smeserver-snort.
Do you use 'https://<smeserver>/server-manager' to access server-manager, or do you use 'http://...' and trust the ReWrite rule to send you to 'https:.'?
Can you get to server-manager if you do this:
1. c:\path\to\putty.exe <smeserver> -P 2222 -L 980:127.0.0.1:980
2. browse to
http://localhost:980/server-manager
(if this works but https://... doesn't, there may be a problem with your ssl certificate).
Normally, straight server-manager http errors should be logged in /var/logs/httpd/admin_error_log.