Double-clicking your HydraCALC icon makes nothing happen.  You do not see HydraCALC open onscreen nor on your taskbar.  HydraCALC does not open other than in the task manager.

If you open your task manager and see the app "Hyd_prog.exe" open but not on your taskbar, then the application is stuck in purgatory.

Another possibility may be that it's on the taskbar but not on your screen.  That means it's open but offscreen. 

See this HydraCARD to fix that issue;  HydraCARD 565 only AFTER you've checked on the solution below.

Blacked out cells in Head Selector dialog

Calc process stops at step 1a.4 and Runtime Error 5 displays

Could not find file, or Unrecognized database format 'C:\HES\HydraCAD\Ver50\AutoListElements\Data\Setup\mdbtrans.mdb'

When trying to open Add Device, nothing happens and Revit just hangs up / appears frozen.

No error, but after clicking Setup, Revit freezes / stops working, and nothing else is able to be selected.

When selecting "Retrieve Part Num/Desc <" from the sprinkler Block Definition window, you get the error;

Error: bad argument type: stringp nil

When attempting to insert a pipe dimension you get the error "Invalid option keyword"

And the command prompt says "Enter an UNDO control option" with the dialog showing your choices

When starting the Pretty Pipes command, you get an Unhandled Exception error including 'Could not find a part of the path'

Another error may mention blockoffsets.dat

When opening AutoCAD, you get an error saying "a software problem has AutoCAD to close unexpectedly"

Upon reviewing the file it's crashed on, it shows HCAD_MAKEFIGHANGERLIST as the culprit