If I'm correct, then your box has frozen since a recent retune and is possibly (but not necessarily) displaying error codes 07 and 09. if this is the case then there is nothing you can do by way of a reset to remedy the fault.
If so, a number of Sagem models have shown incompatibilities with the recent Freeview retune, and your model (PVR 6280) is specifically listed as one ofthose affected (seehttp://news.bbc.co.uk/1/shared/bsp/hi/pdfs/01_10_09_freeview2.pdf
). Sagem are not the only ones, better known brands had problems as well.
The official advice is to contact Sagem directly on 0845 0900 316 to find out if they've scheduled an OAD (over air update).
If I'm wrong in my initial assumption and all you wish to perform is a straight system reset for any other reason then all you do is to switch the box on and then unplug it from the mains and from the aerial, leave it for five minutes to allow internal capacitors to discharge, and then refit the aerial plug and reconnect the mains power to it.
I hope that you've foundmy posting to be of use and ask only that you return the favour byrating my answer.