If you have a flat ribbon type of lanyard, cut the lanyard and then stitch the break back together with thread. It will be strong, but not strong enough to strangle you.

If you have a paracord necklace, you can use the stitching method or use heat shrink tubing to connect the two pieces since both the heat shrink and the paracord are round.

I've done the stitch method and it worked well - breaking strength was somewhere 50lbs. I've never tried the heat shrink tubing method, but I've read about this method being successful for others. If you try the heat shrink, let us know how it goes, I'd really like to hear more about it.