I have recently clean installed MATE 15.10 on a Lenovo X61s laptop.
WiFi works fine when I boot up but if I leave the PC on standby overnight, when coming out of standby the WiFi icon (top right) shows disconnected and have tried -
1/ Soft Switching on and off
2/ Hardware switch WiFi on & off
3/ Suspend and return
The only way to get it working is to shutdown & restart
Seems similar to issue @Need help, wireless issue on Lenovo T61
Thanks
Phil
This is exactly what happens to a friend of mine too,if you find a solution please leave a reply. 
I am having a similar issue with wifi when using my laptop after being in standby. It will work but usually very poorly for awhile. Then after a few minutes it disconnects and can’t see my home network. If I disable then re enable networking and wifi it sometimes can find the ssid and reconnect but other times I have to do a reboot to get it working as it should.
Same here, X61s (forget which wireless hardware) running 15.10. It doesn’t matter how long the machine is on standby, I lose the connection, but turning the switch on and off sometimes resolves it.
I’ve been having unpredictable network problems since 15.10 - some I’ve mentioned here. Some of this could be happening because I too put my computer to sleep when it’s not going to be used in a while.
This command should work:
sudo service network-manager restart
– or –
sudo service NetworkManager restart
It’s far from a solution, just a workaround that kicks it back into place, instead of restarting the entire computer, where it seems the networking is a bit buggy in 15.10.
If you have the RTL8188EE wifi adapter and are running an older Linksys router the solution I found in the Ubuntu forums (for my HP Slimline w/RTL8188EE adapter) was to replace the router with a newer model, I replaced my old Linksys router with a new Netgear router and the issue went away, I’ve had no trouble since. I did this after following one poster’s struggle with software solutions that didn’t work and him doing so with the issue being resolved. This might be happening with other wifi adapters too, I don’t know. I would suggest you check in the regular Ubuntu forum for your due diligence before spending money. I can’t locate the link where this was referenced or my comments how replacing the router fixed it per the previous poster’s post, evidently others aren’t finding it either because I see a few new topics with the same issue that I resolved by replacing the router back in August/September
I hope this helps
this is one of the common problems for every internet user. I also faced this type of problem 1 year before with my DELL PC. I got a proper tech support from netgear tech support canada.