Click a button. Maybe it has gone into power saving mode. Wireless mice go to sleep to save the batteries which run down fast, much faster than the ones in the keyboard. My el-cheapo Trust wireless "desk set" has this really annoying mouse that hibernates after about 30 secs of inactivity, and you have to click a button to wake it up. It came with 2 NiMH rechargeable AA cells and a power/charging wire that plugs into the wireless "base station" but even in this mode it goes to sleep. It is a pain in the @ss. Also it is ugly and looks stupid on the desk. I hate it and I went back to my USB mouse with a wire on it and I just use the keyboard, which I really like, in which the batteries last about 6 months.