A tile that rocks under a step at the bathroom door is a trip waiting for the next night walk. You can often fix a single loose tile yourself, but a few checks tell you whether that is the right call before you pick up a tool.
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Stop first if the floor is wet or soft
Press gently around the loose tile and look at the grout lines. If the surface under the tile feels soft, gives way, smells damp, or stains when dry, do not re-set it. That is a water question, and it goes to a licensed plumber or leak repair before any tile work. Tile laid over a moisture problem will come loose again, and the trip will be the same.
Count the tiles that move
One tile that rocks, with solid tiles on both sides, is a small job. If several tiles move, or the loose area runs across the doorway threshold, the work is bigger and the floor underneath may be the cause. In that case a tile setter should look at it, because the fix may mean lifting more than you planned.
Check whether the tile can come out cleanly
Ask yourself whether the tile is cracked, chipped, or glued down hard. A cracked tile usually has to be replaced, and a glued tile that will not lift without breaking its neighbors is a job for someone with the right tools and a spare tile on hand. If you can lift the tile, clean the old adhesive, and set a new bed under it without touching the surrounding tiles, you are in do-it-yourself territory.
Compare the two routes on paper
Write down what you would need for each route. For the yourself route: the adhesive or mortar the tile needs, grout to match, a spare tile, and a Saturday you can give to curing time. For the hired route: a written estimate that states the number of tiles covered, whether the floor underneath is checked, whether the old adhesive is removed, and what the warranty covers if the tile loosens again. Pick the route where the paper is complete, not the one that is cheapest on the first line.
One rule for the night route
Whichever route you pick, do not let the repair leave the doorway step uneven overnight. If the tile is still curing, put a clearly visible, flat barrier in place, and make sure the light on that step works before the first trip after the job.