Headache: Suddenly no signal from WAP

Hi, I got a problem giving me major headaches 8O . Ill be brief:

Acer Notebook wirelessly connected thru a Wireless Access Point (from Devolo) that's connected to a router (from Netopia);
-> Worked for months.
-> But, I had to disconnect the WAP -> instant trouble.
-> Now it almost never connects anymore (but sometimes it does!), even when there's only a two meters between them.

-> PROBLEM: the notebook just doesn't pickup a signal from the WAP, as is seen in the standard Windows XP Wireless Connections-overview.

How can I solve this? If you require a more specific question:
How can I check, without borrowing another notebook, if the WAP still emits a solid network-signal??? Any help would be greatly appreciated.