Xsara wont start

Try replaceing starter relay with another of your relays. make sure they have the same part number on them if you switch them and it starts replace relay with new.

Do a scan of the car computer, go to a garage and have it connected to a code reader, that will tell you what the problem is.

crank / cam sensor faulty

starter motor on its way. also check and clean battery terminals and connection to battery.

If it's the main Fuse, rated above 40Amps, then usually only two components are directly connected to this circuit. The starter motor and the alternator. Both can get critical short to ground, that may blow such fuse.The thick battery cable goes directly to the starter motor and a smaller high amp wire is looped from there to the alternator. Third less likely would be main supply to ignition key switch.

Sounds like ignition timing or needs a new battery/alternator.
Have your system tested for free at Autozone.

It sounds like you have a couple of problems going on. First if your car wont start it could be the starter but if there is no noise you may have an ignition problem. Ignition problems usually occur in the steering wheel column. Try tilting your stearing wheel up and down a couple of times. If that does not work it might be your starter if it is you can get under your car and arc it. Or it could be your altenator but the only way to test that is if you take it off and take it to napa or something . I hope it is your altenator because it is usually a quick and cheep fix. good luck.

check all your fuses and replace any blown ones, especially the high value ones

sounds like a bad connection,it wont heat up the solenoid or the starter,check your hot and negative cables from battery to starter first could be the starter itself, ggod luck

assuming the engine run good and nothing wrong with it, check ignition relay, can get them from junk yard if need to replaced them and will cost you barely nothing. my guess would be your ignition switch, it tend to wear out over time and loose contact. the unit is not cheap to replace and is common to some vehicle over time, some has better luck than others; this part run some where around 340-350 bucks. you can try to take ignition unit apart and jump the wire to see if that the problems. ignitions switch is like a master switch, if it faulty, it will not turn anything on, it seems like the battery is dead or something is disconnected. try this first before you decide to take ignition column apart, the next time it wont start, try to jiggle, wiggle and play with ignition unit with the key is in it, turn they key back and forth, try until the car turn on or start and see if that work. you have to play with it for awhile, if its faulty, it will also be stubborn.