Hi @maarmuis,
Usually 1 beep indicates a successful POST on HP/Compaq laptops.
Do you get the “splash” screen displaying on startup, before the “beep”?
Try a power refresh and check if this resolves the problem.
Remove the charger (if connected) and the battery from the laptop.
Press and hold the Power button for 30 seconds to drain any residual power from the laptop, then release the button.
Reconnect and switch on the charger to the laptop (leave the battery out at this stage).
Try to turn on the laptop.
If it starts OK, allow it to boot all the way to the Windows desktop (if you have Windows installed), wait for the HDD activity to settle a bit, then shutdown the laptop in the normal manner.
When it has completely shutdown, switch off and disconnect the charger from the laptop.
Reinsert the battery into the laptop.
Reconnect and switch on the charger to the laptop.
Try to turn on the laptop.
If it starts OK, allow it to boot all the way to the Windows desktop (if you have Windows installed), wait for the HDD activity to settle a bit, then check the charge status of the battery. if it is charging allow it to fully charge before switching off the charger and disconnecting it from the laptop.