Here's one very likely reason, Red Hat 7.3 is ancient. Support ended for that years ago, it shouldn't be in use anywhere for any reason. That would be like finding somebody on windows 3.1 or 95.
Beyond that, if the hardware it is running on is as old as the OS, things are probably breaking down.
Also, if this is a server, it may not be hanging at all. Is there a line that says something like:
?
Often servers will boot to a command line prompt, and not start any pictures.
The list of actual reasons for a freeze are as long as this website. You could have any number of software issues, kernel issues, hardware issues, etc.
Giving us no clues as to the problems and asking for all the possibilities does neither us nor you any good. Help us to help you - when was the last time the machine was used correctly? Was any new hardware or software added? Has any hardware been acting up?
Peace,
JimBass