Q: OS X 10.10.4

What does Wireless Diagnostics reveal?

About Wireless Diagnostics - Apple Support

This is the Mac Mini desktop forum. I requested your post be moved to the MacBook Air laptop forum.

Go to > Solutions for connecting to the Internet, setting up a small network, and troubleshooting - Apple Support and work thru the "internet connection and troubleshooting" section. Plus if you move around and connect to different locations, also work thru the "Using your computer in different locations" section.

Try System Preferences/Network - select the Wi-Fi, then click Assist Me, and then select Diagnostics.

Did you power off and then back on your router?

Does the Mac connect to other networks?

What encryption are you using on the router?

Try changing it.