Sorry for the confusion.
Here's what happened:
I installed Ubuntu 11.04 ( kernel 2.6.38.10 generic) on a Toshiba Satellite A100-250. Wireless was extremely slow using the ath5k driver, so on suggestions from the forum I installed madwifi. This worked fine.
Wifi stopped completely after a suggested update, which had installed a new kernel 2.6.38.11. Looking at the network manager when this kernel has started up, only the ethernet connection is running, no wireless at all.

Frankbell suggested I change the kernel call in grub, but this is too intimidating to me.
So I can either:

1. Revert completely to the previous kernel,
2. Try to fix the current kernel to support wifi,
3. Manually choose the previous kernel on startup.

If the current kernel has important fixes compared to the previous one, option 2 would be preferable.

Ok, That clears things up a bit. I don't understand what the problem is about reverting to the previous kernel. Normally kernels (called linux-images) don't disappear unless you explicitly remove them. When you boot up, don't you see a list of kernel options, each one in normal mode or recovery mode, to choose from? I don't see why editing grub should be necessary unless you did something drastic.

2nd point: 2.6.38 is nowhere near being the latest kernel version. There's 2.6.39 and then 3.0 came out to much brouhaha a while ago. I'm running 3.0-3.dmz.2-liquorix-amd64 right now. So you have two ways to go: backwards or forwards. If you're unhappy with your kernel's performance then change it. It's not a big deal.
ciao,
jdk

Thanks for looking into this issue.

When starting up grub indeed offers choices. Current kernel: 2.6.38-11, and recovery mode.
Going back to previous it's 2.6.38-10 and recovery mode.

How to change it (either forward or backward) is what I don't know how to do.
I understand that the kernel call can be changed in grub, but honestly I hesitate to go that road.
On the other hand, how to get wifi going in the current kernel is also beyond me.

Wow! Ubuntu seems incredibly complicated. I use Debian and when I see the menu I just use the arrow keys to move upwards or downwards. When I arrive at what I want, I just hit <CR> and voilà I've got the kernel I desire. Doesn't Ubuntu work this way?
But that's what Grub does! at least one of the things it does. It allows you to choose which kernel to boot up in. Believe me, it's not a big deal. You're just selecting an item on a menu. If you're not happy reboot and select a different kernel.
ciao,
jdk

No, Ubuntu's not any more complicated. It's exactly the same thing (It's Debian based). You can select which you need as explained before. That's what I do now. But I have to do this manually, or it will default to 2.6.38-11.
I hesitate editing the grub, that's what I mean.

So the questions remain:
1. How to get wifi going in the current kernel (2.6.38-11)
2. How to revert completely to the previous (2.6.38-10) (Not just manually choose it on startup)

Grub normally defaults to the first menu item (typically the most recent). Selecting another kernel will not change the default. In the old Grub days you could edit the file menu.lst and set the order (and thus the default) any way you wished. I don't know how that's done in Grub2. As for your questions, I think they have been answered:
1. For some reason you can't. I would suggest getting a more recent kernel e.g. 2.6.39 or 3.0.
2. Remove 2.6.38.11 That's it.
ciao,
jdk

O.K. I'll give that a try.
Thanks for the assistance.

It's easy to get grub to use the kernel you want as default:

Use a text editor to open /etc/default/grub as root or use another editor.

Start of file:Change GRUB_DEFAULT=0 to GRUB_DEFAULTLT=2, assuming the correct kernel is 3rd down from top.

Then run update-grub like it says.

Must do some reading on use of vi before attempting this, I guess.