Kevin is likely right. if it is not working to determine what to replace it with will require you to post the part of a schematic of where it is used, together with information about the likely voltage, current, and application, i.e. high frequency switching, or mains, for example, cheers, Richard.
Sidac is a bi-directional voltage triggered switch very similar to the diac, and it belongs to the thyristor family ‘ this is not common like diac. SIDACs have higher breakover voltages and current handling capacity .