Why is SiO2 a solid whereas CO2 is a gas at room temeperature?

The main factor influencing a given compound's state is the strength of intermolecular forces - those need to be larger than the thermal energy for a compound to be solid/liquid. For CO2 the only type of interactions possible are the weak Van der Waals forces, whereas in the case of SiO2 the solid is stabilised by the presence of strong covalent linkage all throughout the crystal lattice.