Loose sill blocks are not that uncommon. They are (usually) the result of either an inadequate amount of mortar underneath them during initial construction, or deterioration and the resulting lack of bond of the mortar that was used. Freeze-thaw cycles will always tend to work on members like this and their bonding material, and over time can result in the members breaking free.
A proper repair would consist of removing the blocks and all mortar completely, cleaning things up with a wire brush to promote good bond of the new mortar. Re-install the blocks using proper mortar for your climate, and don't forget to renew/replace the flashing if the stuff that's there is damaged (or missing). Using a flexible, self-adhesive flashing will ensure that the new mortar will have a tight bonding surface.
A temporary repair of just removing any obviously loose mortar from the visible top, vertical and underside joints, followed by placing a few beads of good polyurethane caulk everywhere the mortar was removed. Doing so will buy some time before undertaking major re-work as initially described.