No boot device found on my hp Pro book6465b

Did you make any major changes like wipe the hard drive? A lot of the time, a sudden boot device error means the hard drive has failed, especially on an older system like this. Listen to the area where the hard drive lives - see if you hear it go click, click, click continuously or if it is grinding (should be on the right side of the palmrest near the trackpad) - if so, bad drive; no diagnostic SW needed. If not, run the HP diagnostics and check the hard drive specifically - for this model, you need to pick the Additional Recovery Support option and select 2017 and prior due to when this model came out. You may also want the Vision diagnostics in case you need the AMD Vision ones for some testing, but the standard HP should work.

If the original drive has failed, install a 256-512GB SSD to replace the original drive. If the OS has corrupted you can try your luck at a reinstall, but if you run into any additional issues with the drive it’s time to replace it.