Why doesn't my Macbook detect network after sleep?
Clear the network out and re-setup it.
Otherwise, I've run into this issue before, what brand of router are you using? And does it happen with other networks apart from this one?
Related: Why doesn't Airport reconnect after sleep or shutdown?
Apple claims to have addressed this issue with the OS X 10.7.3 update which, amongst other things, has a fix to:
Resolve a Wi-Fi connection issue when waking from sleep
Ars Technica reports that some people have had problems with the update but they are resolved by reinstalling using the Combo Updater.