The right ways to regain access
1) Call a licensed locksmith
Have photo ID and proof of tenancy/ownership ready.
Ask for non-destructive entry first (picking/decoding) and a quote for rekey or cylinder replacement afterward.
If the cylinder is damaged beyond service, the locksmith can remove and replace it on the spot.
2) Work with the property manager or building/security office
Many rentals/managed buildings require authorized lock changes and may send their preferred locksmith.
Set expectations: who pays, allowed hardware, and key control (how many keys, who can duplicate).
3) If you can already open the door from another entrance
You can replace the cylinder without cutting anything once the door is open:
Identify the format
Euro profile (figure-8 cartridge, one fixing screw in the door edge)
Deadbolt cylinder (key outside, thumbturn inside)
Mortise cylinder (threaded brass body)
KIK/KIL (key-in-knob/lever core)
Basic removal (non-destructive, door open, with the key):
Euro: Remove the single fixing screw on the door edge; turn the key a few degrees to align the cam; slide the cylinder out.
Deadbolt: Remove the interior trim/through-bolts; withdraw the exterior cylinder with its tailpiece.
Mortise: Back out the case set screw (if present); insert key and slightly turn; unscrew the cylinder.
KIK/KIL: Remove the knob/lever trim; release the small retainer; pull the core.
Install the new cylinder (match size/cam/finish; for Euro, specify Outside/Inside lengths so the exterior sits flush). Test with the door open first, then closed. Consider upgrading to restricted keys or anti-snap (Euro) if appropriate.
4) If the key is lost
Ask a locksmith to rekey the existing cylinder (cheaper and preserves hardware) or install a new one keyed to your system.
For frequent rekeys (offices, rentals), consider interchangeable cores (SFIC/LFIC) so you can swap cores quickly.
Preventative tips (so you don't end up here again)
Keep a spare key off-site with someone you trust.
Use code-cut replacement keys (not copies of worn keys) to maintain smooth operation.
Clean the cylinder periodically with non-residue cleaner and a dry lubricant (graphite or dry PTFE).
On egress doors, ensure the inside remains key-free (thumbturn or exit device) to meet life-safety rules.