Fresh line will not come out, or it spits a tangle and stops, and there you are in half-cut grass holding a tool that will not cooperate. On Ryobi ONE+ trimmers this is almost always the spool or the line, not the motor. Work through these in order and you will usually have it cutting again in a few minutes.
First, check which head you have
Many Ryobi cordless ONE+ trimmers use an auto-feed head, and Ryobi says bumping an auto-feed head can damage the trimmer and void the warranty. On those models, release the trigger, wait two seconds, and press it again to advance line, or remove the battery and press the black button on the head while you pull the line. Only bump the head on bump-feed models. Check your manual for which head yours is.
First, the line has probably welded to itself
This is the most common cause by a wide margin. Friction generates heat at the head, and the heat fuses adjacent wraps of line into a solid lump that cannot pay out. Pop the head open, find where the wraps are stuck together, free them, and pull a few inches of line out past the eyelet before you close it up. To stop it happening again, do not hold the bump on a fast-spinning head, and wind the spool a little less tightly.
You might be running the wrong line size
Ryobi heads are built for a specific line diameter. Go too thick and it jams or refuses to advance, and you can chew up the spool or spring in the process. Check what your model is rated for. Ryobi ONE+ trimmer heads take either 0.065 inch round or 0.080 inch twisted line, so match the number your head calls for rather than whatever is on the shelf. Twisted versus round line matters far less than getting the diameter right.
The line is cross-wound or wound backward
Follow the wind direction shown on the spool (check your manual), because ignoring it almost guarantees a feeding problem. Rewind so the line follows that direction, keep the two strands in their separate channels, and wind them tight and parallel with no overlap. Cross-wrapped line snags on itself and stops dead. Seat the loose ends in the retaining notches so they do not unravel before you reinstall the spool.
Clear debris out of the head
Grass, mud, and grit pack in around the spool and the retainer and physically block the spool from moving. Take the spool out, clean everything, and confirm the spool slides and spins freely in the housing before you put it back.
Check the bump knob and its spring
On a bump-feed head, the spring under the knob is what releases line each time you tap. If that spring is weak, kinked, or broken, no line comes out no matter how hard you bump. Inspect it and replace it if it has lost its tension. While you are at it, only bump on hard, flat ground. Soft dirt absorbs the tap and the mechanism never triggers.
If yours is an auto-feed head
Some ONE+ trimmers use a Reel-Easy+ bump-feed head, and others use an auto-feed head. Check your manual for which you have. If an auto-feed mechanism itself has failed, you usually replace the whole head assembly rather than try to rebuild the internals, and it is an inexpensive part.
Knowing when to stop fixing and start replacing saves time. A spool that is cracked, warped, or melted, or a spring that no longer springs, calls for a new spool or head, and both are cheap. But if the line, the winding, the size, and the spring all check out and it still will not feed, swap the head and move on.