Your question does imply that it has been working but went back to cold after a few days - hence my answer.
You need to check the heater motor resistor / thermistor pack. Problems with this unit can 'kill' all but the full-on speed.
IF the layout in a '97 car with aircon is generally the same as a '96 without, (anyone know different?), - then you will need to get under the passenger side dashboard to gain access.
Removing the Glove box tray helps a lot.
If you look back over this thread and the other one I posted about flushing a Laguna heater, you'll see a few pics of this general area -noting the air intake pipe running from the left hand side of the footwell (also where the blower motor is) - across the footwell to the heater mechanism.
The Resistor pack sits on top of this large bore plastic pipe and is recognisable by the cables going to it. I have highlighted it in one picture. Its here deliberately to put the resistor
into the heater's incoming airflow - a sort of safety valve in case the airflow is blocked.
Unplug the cables from the top and when free, twist the resistor pack through about 20 degrees and pull out. If it won't twist- check for a single locking screw.
Examine the pack - following the guidelines in this thread. You'll need to take of the protective metal cowl - CARE! - and check the integrity of the resistor
and also the thermal fuse with a continuity meter or similar.
If the thermal fuse has blown (v. common) you can get a spare from Maplins , 110' C I believe. Cut out the old one (spot welded) and attach the new. Soldering is hit and miss (its a THERMAL fuse !) so mechanical attachment is safer (this thread suggest metal inserts taken from a chocolate block type cable connector - sounds sensible to me.
If any of the resistor
are broken you'll need to call-it-a-day and get a new one from Renault (unless your good at hand winding resistor
Before re-assembly make sure the air pipe-work and blower motor enclosure are free from debris - usually leaves and twigs - as a restricted airflow might be the reason the fuse blew in the first place.
Blower motor is removed in a similar way....remove locking screw(s) - twist to release and pull out. Bit of a fiddle, but not that awkward.
If this doesn't help or the resistor pack is different on an air con equipped car then I can't help further.