Without more information on the nature of the problem reboot is the best answer. Also if you are still having issues after the reboot turn off password protection and see if they can connect. THIS IS NOT THE WAY YOU SHOULD LEAVE IT! Sometimes the authentication protocol is not compatible with a device. If turning off password allows them to connect you know it's an issue with the authentication type.
Install this app on a laptop. Comes for Windows or Apple. Walk around and do a site survey to test signal strength at different areas at your location. See if they still have the problem in an area with a strong signal.
p.s. You will likely note many other networks from neighbors. If your network runs 2.4Ghz, note some wireless home phone systems use that and it may cause problems, too. May need to switch to another band.
Also, changing the channel within a band could help, based on neighboring network traffic.