I’m afraid that doesn’t tell us much. There’s really a lot more information we’d need, but a few things that come to mind off the top of my head are:
What hardware (motherboard, CPU, RAM, drives, controller if any) are you using?
Which version(s) of TrueNAS are you using?
Were you able to boot the TrueNAS installer?
Onto what device did you install TrueNAS?
Did installation complete without errors?
Is the new boot device–the one you installed TrueNAS onto–still connected to your computer?
Is it set as the boot device in your BIOS?
TrueNAS works for many thousands–if not tens of thousands–of users. There may be a bug that’s preventing it from working for you, you may be doing something wrong, or perhaps both, but it certainly isn’t the case generally that it doesn’t work.