selecting more than one entity

Hi all,

For some strange reason i'm suddenly unable to select more than one entity just by clicking with my mouse.

for eg if i select a line and try to select a second one ,the first automatically is deselected.

i'm only able to select multiple entities using the" cross windowing" or 'fulyl windowing' an entity.

this is only a recent occurence  and i suspect i may have inadvertently pressed some key.

any help?