single user mode to multi-user mode (vice versa)

[Kerry Ong <kerrykong pacific net sg>]

Greeting

On one occassion, have encounter unable to do remote telnet session to
RedHat OS.   There is no light indication on the system HDD is running.
After a cold reboot, it changed from  multiuser-mode to single user
mode.  Able to access the system with same root password.

Appreciate your expert advise :
1)What is the possibilities causes of RedHat OS hung status
2)How to change single user mode to multi-user mode (vice versa)
3)Any console command or maintenance can be done to avoid such situation
to re-happen ?
4)What is the constrait of single user mode and advantages of multiuser
mode ?
[i couldn't locate any information in this area]


Thanks
Kerry