Star magnolia lost all its flowers after repotting

~was it in a pot before the move or in open ground?

is any of this relevant~from RHS site
Although it is undesirable, sometimes mature plants need to be lifted and moved to new locations. Risks can be minimised if site preparation and basic procedures are followed

Moving any established tree or shrub is risky, regardless how carefully the work is carried out. Any plant will suffer stress when uprooted, which often results in growth being checked. Shrubs such as Magnolia, Rosa, Cytisus, Cistus and Elaeagnus particularly resent root disturbance and can be tricky to move. Young plants transplant fairly well if carefully lifted at the correct time. More established plants will suffer greater stress and require advanced preparation. Consider using a specialist contractor to move mature or large numbers of trees.
Time

Move deciduous plants from late October to mid-March. Move evergreens in October or late March, when the relatively warm soil will allow roots to re-establish quickly.
Plant preparation

Mature specimens require preparation a year in advance. During November to February, while plants are dormant dig a circular trench one spit wide parallel with the branch spread. Back fill the trench with sharp sand to encourage fibrous, feeding root growth which will help the plant to re-establish quickly.

Additionally, prune out approximately one-third of the plant’s wood.
Site preparation

Mark out the expected root spread plus an extra 50cm (20in) and dig the area to 30cm (12in) depth. Fork over the bottom of the hole, adding some organic matter.
Lifting and moving

Water the soil well the day before moving.

Determine the extent of the root spread with exploratory digging. Loosely tie-in branches before lifting. Lift the plant with as much rootball intact as possible. Keep roots covered to avoid desiccation.
Storing before planting

Pack the rootball with organic matter and wrap it in sacking, before placing it in a cool, shaded spot. Keep the plant well watered.
Replanting

Place the plant in the hole, checking that the roots can be spread out fully. Where necessary, adjust the size of the planting hole. Use the old soil-mark on the stem of the plant as a guide to the correct new planting depth. This will prevent replanting too deeply (one of the biggest killers of all plants). Equally, planting with upper roots exposed will damage most plants. Firm around the plant carefully to eliminate air pockets as you fill in the planting hole.

Larger plants and those placed in windy sites may require staking or guying for the first year after planting to prevent wind rock.
Subsequent care

Water-in thoroughly after planting and continue to water during dry spells. Apply a thick mulch of organic matter such as chipped bark, which will help conserve moisture and suppress weeds. In spring, as growth begins, apply a general fertiliser.

THANKS so much. Actually it came in one of those black plastic pots; the ones they are sold in- I was careful when I re-potted not to disturb the roots. When will it bloom again?

If it was in flower when you repotted it that was probably the problem. My advice, too late I know, would have been to place the plant and its plastic pot inside a more attractive container for this year. I would then have repotted it after flowering.
It will flower again about this time next year, whenever they come into flower in your area, and should be OK.
I see that you say that you were careful not to disturb the roots. You may want to let the plant settle down over the summer and then unpot it, tease the roots out and then repot it. The roots are probably pretty tightly packed and in peat, by teasing them out into a decent potting mix you will have a happier plant.

if you have had a lot of windy weather then it could be due to water stress. but as bulbaholic says next year it shold be ok.

you just shocked it at a bad time.very unnatural for a plant to be moved so it shocks it to a lesser or greater degre when ever you do it.i had a fatsia when i moved in a pot and didnt know any better.all i did was move it and it died without repotting yet i have a realy healthy specimin growing in my garden now in the same place.

Thank You all sooo much. I feel better already...and will do the repotting thing after the summer! Ciao

your welcome mate