Assuming you've checked the compatibility matrix for the version of WA / bitness etc,
Your earlier Apache was 2.2, thus you'd have in httpd.conf
LoadModule sm_module "/webagent/bin/libmod_sm22.so"
After upgrade your Apache is 2.4, thus you should (only) enable in httpd.conf
LoadModule sm_module "/webagent/bin/libmod_sm24.so"
If you are knowledgeable on what to change, then you could do this manually. Otherwise just re-run the configuration wizard (you can skip host registration and only concentrate on Agent configuration).
Also remember to source the ENV file /webagent/ca_wa_env.sh either in setenv.sh OR manually before starting Apache.
If you also happen to look at the Apache error logs, you should see some initialization errors, if something is not configured correctly between Apache & WA.