How long has this problem persisted?
Have you powered down your wireless modem, and then switched it on again? The other day my smartphone couldn't detect any Wi-Fi signal from my Netgear modem, even though the wireless light was illuminated on the modem. After I powered the modem down for a few hours, and then switched on again, my phone found the signal immediately. The phone was displaying other nearby Wi-Fi signals, so I know the fault was in the modem.