A common problem on 240s is with bonnet release cables that either break or seize leaving the bonnet firmly closed and apparently unopenable. There are two ways of access.
The first is to try lifting the grille and provided there is clearance for the bottom lugs the grille can been pulled forwards at the bottom and then down to remove it giving access to the latch.
The way that does not risk damage to the grille is as follows. Underneath the car you will see a piece of plastic clipped above the spoiler:
Unclip this and pull it back to allow access for your arm:
You can then reach up inside with your arm to the catch:
You can either pull on the cable or press on the other side of the swivel (push towards LHS of car) Finger in the picture is in correct position for pushing lever to open.
If the cable is broken the cheap fix is to fix a length of electric cable to the catch (where the finger is) and push this out through the grille. This can then be pulled towards the LHS of the car (to your right as you face the car) to release the catch without needing to go back in from underneath.
The instructions in this tutorial will work in the following models / years:
-1974, 1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992 and 1993 Volvo 240, 242, 244 and 245
-1974, 1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982, 1983, 1984 and 1985 Volvo 260, 262, 264 and 265