single beep on start up

Help us help you. The more complete the description of your problem is (which exact MBA you're using - there are many versions going back to '08, what OS you're using -there's a lot of those too, what Application you're using there are many, a full compete description, the text or screen shot of any error messages) the more assistance we can provide, and, the sooner we can provide it.

In this case some history (was it dropped, crushed, wet or otherwise physically damaged) about the unit right before the beeping started would be very helpful.

You appear to have a RAM issue. It could be bad or the solder joint may have cracked. Since RAM for these machines is soldered to the logic boar this is not a good candidate for a DIY repair project. De-soldering and soldering of computer logic board and components requires special tools, solder and lots of practice so as not to damage the logic board with too much heat.

Depending on the history and what else may need repair only you know if it is ecoomically feasible to take the machine to a computer/electronics shop where someone would have the tools and skills to test and re-seat the RAM.

If this answer is acceptable please remember to return and mark it.