First ensure that the main router's wireless is working correctly and that any nearby wireless devices connect without issue.
Second, turn on SSID Broadcast. Hiding the SSID is not really a meaningful security measure.
Third, reconfigure the Belkin N300 as if new.
Do you have the User Manual?
If not:
http://cache-www.belkin.com/support/dl/MAN_F9K1111v1_8820-01079_RevA00_N300_RangeExtendr%20(2).pdf
Not sure what the technician may have done per se. However, very good chance that your Range Extender may have gotten reset back to its original factory settings.
You can also use the status lights to troubleshoot (see Page 7 of the User Manual).
And do be patient. May take a few minutes for the router and range extender to...