Troubleshooting is not for everyone
Sounds like you panicked(and I don't blame you) and sometimes this can cause more issues than it solves. Here is a rundown on how I would approach this:
1-If Pro Tools opens; check the hardware setup and Playback Engine to make sure the HDX card is selected.
2-if #1 is correct; (on a Windows machine)open Windows Sound Properties and make sure that Windows is using the onboard sound card as the "default playback device"(NOT the Avid hardware). If you are not running Windows, then you can see why details matter
3-run Digitest. If that comes back with a failure, it probably will give some detail on what/why. If it comes back all good, move to 4. Fail, move to 5
4-uninstall Pro Tools and the HD driver and re-install(which on Windows, requires you to open the Apps and Features window and then open the Programs and Features window, find Pro Tools< right-click and select "Change", then follow the prompts, remember to check the box to also remove the HD driver)
5-Digitest failed, is the HD IO lit? Have you reseated both ends of the digilink cable(gently and carefully)?
Did you install any new software for the last time it worked(even if the software is not related to Pro Tools)? Do you have a backup drive image of your system drive(you should)? I keep a clone of my system drive(done at a point when the rig is working great) so I can swap drives.
Beyond this, are you getting any error messages or any indication from the computer that something is wrong? If things are still not working, come back with every little detail of the entire setup(read the
Help Us Help You link near the top of the page) along with details on exactly what you have done.