Signs You Can Repair It
If the key is simply hard to turn, a spray of graphite lubricant (not WD-40) might fix it. Loose handles can often be fixed by tightening the internal screws.
Signs You Must Replace It
If the lock cylinder is completely jammed, the key has broken off inside, or the internal spring mechanism is snapped, repairing it is futile. You must replace the lock cylinder or the entire handle.
Cost of Replacement
Replacing a standard HDB room door cylindrical lock costs around $50-$80. Main door mortise locks are more complex and range from $120 to $200.