It may have simply been turned off. Where the setting is for this varies
from system to system; on some older systems it can only be disabled in the
BIOS. On some newer ones, there will be a software control. On
Thinkpads, for example, the ThinkVantage applet set contains this. Check
the manual and help files for your system.
Might help if you mentioned what the system is, by the way.