I would agree with what
claudine has already said - in that it very much depends on what type of plant it is but would add - that it also depends on the length of time the plants spend in freezing conditions.
Most semi - half hardy perennial plants and trees - even some succulents can withstand short periods of freezing temperatures even ones as low -5C - without doing too much harm - other than leaving some brown tinged or melted foliage - which although the plant won't look very attractive - it won't have been enough to kill the plant.
It is when you have regular prolonged periods of freezing weather for months on end that normally causes non- hardy plants to die - but saying that - my 6ft Aloes were once attacked by extreme weather for six weeks or more when the temperatures plummeted to well below - 15C with a wind chill of double that and survived - even though the tops had melted - the roots were fine and they are now back to their former glory
However if you know that a freak cold snap is coming - it is best to do as you did and protect as many plants as possible - which can be be done by bringing them temporarily undercover and covering those that you can't move with horticultural fleece - which is preferable but as
Pat - has already pointed - can be done by covering them with soda bottles, egg cartons and even newspaper - but I would advise against using plastic bags or sheeting - for the simple reason that when you place plastic over a plant it tends to gather moisture on the inside - which over time collects and then freezes at night - which of course is just as fatal - if not more so than leaving a plant uncovered.