why wireless connection not working?

We live in Apt. Here is the configuration of the network in our apt.

Cable Modem -> Wireless Router.

Laptop A connects to the wireless router in a wired ethernet cable;

Laptop B connects to the wireless router wirelessly.

Everything works fine. Then suddently two days ago, B cannot access the Internet, or the connection of B is VERY slow.

A's connection is fine.

No change is made. Why is that?

Thanks a lot.