My endo gave me two pens. I started a little more than a week ago. I found Tim's advice compelling and dropped my dose 33%. I had been taking 54 units/day and started with a simple regime 20 units twice a day, 12 hours apart at 6am and 6pm.
And surprisingly my blood sugars responded well. In fact, after 2 days I concluded that I needed to reduce my dose, so I moved down 10% and then another 10% down to 16 units, twice a day. But now my fastings are drifting up so I have moved back up to 17 units/day. I am still adjusting.
I'm not sure what to tell you about the Levemir. It seems strange that it would just stop working. You can develop a reaction to certain insulin's (antibodies) that will over time render them ineffective, but it generally doesn't just happen overnight. I would not immediately suggest that you take my experience as an indication that you should use more NPH. It is probably better to be conservative and think that they have a similar strength. In fact, it might be prudent to try NPH during the day to gauge your response before trying it overnight.
I am still working out the kinks on my Levemir, but it has demonstrated that it can tame my Darn Phenomenon without causing daytime hypos. So I'm probably going to switch.