An air lock is a pocket of air somewhere in the pipework which blocks the flow of water. Normally found on tank (cistern) supplied hot or cold water pipework which is not under mains pressure.
To remove, try this remedy wherever you have a mixer with one supply working but not the other: (Thanks to Softus)
1. Put a sponge inside a carrier bag, place it under the bath filler, and press it firmly upwards with your palm.
2. Turn on the cold tap.
3. Turn on the hot tap.
4. Wait until the gurgling stops (if there is any).
5. Turn off the hot tap.
6. Leaving the cold tap open, abruptly remove the sponge.
If you get some flow, then repeat until you get better flow - this method will almost always sort out an air lock.
Another method you can try is bridging the hot and cold water. If you have a mixer tap in the kitchen it may be you could do it by putting your hand or the sponge over the outlet and turning both hot and mains cold supplies on.
You can also try using a hose between the two - or link across washing machine supplies.
If you have a water-capable vacuum cleaner (Carpet cleaner or Aquavac) you can try sucking water through the offending tap.
Use short bursts for hot water, as you can suck more air in down through the vent which is over the loft tank.