error sending sms via gprs after Crtl+Z

all is working well since 3 days, and in few secondes, no sms can be send.
with a phone I can send sms with the same SIM, but not with the shield gprs.
I up the exemple on the wiki, and nothing.
as the Ctrl+Z is send, there is a big ERROR on control.

where is the problem?
I bougth 3 shield gprs this week, and I’m affraid if the pb is the same after 3 days working.

the problem is now, that I can’t make an ATD or anyting to send sms.

I do an AT&F, nothing change
There is another option for a hard reset to restore factory setting?

There is a hard reset pin called NRESET on the SIM900 which can be connected to ground to reset SIM900. Read up the hardware spec.

thank’s, but I read the spec and I don’t seen anything about hardreset.