F5 not booting up in normal mode

Maybe the HDD days are over? Can you boot the system from another boot location (if one exists?). RHEL underneath TMOS doesn't break by itself, so the odds of a hardware problem are high, unfortunately. Do you have active support? Get in contact with F5 to see what your options are. HW appliance could have an active warranty in which case you are eligible for a replacement unit.