Answer: In general, pruning an old lilac by cutting it off at the ground should cause it to send up numerous new shoots from the roots. It sounds terrible to cut the thing off at the ground, but they regrow quickly because of the large established root system.
This would recreate a shrub effect rather than the tree effect you have now. This is called rejuvenation pruning. After doing that you would follow routine annual pruning methods (remove a few of the oldest stems every year or so and thus continue to encourage new growth from the roots) to keep the shrub growing vigorously.
Alternatively, you could try trimming back the blooming shoots at the top but leave the main trunk as is. You would do this immediately after bloom. This should stimulate additional branching among the existing shoots and cause the top to thicken up somewhat. It might also encourage some growth from the roots which would help thicken the growth at the base as well.
Lilacs are usually pruned only in late spring right after blooming because they then grow the wood which will set the buds for the following spring. Pruning in mid or late summer will remove the next year's flowers. Sometimes this accounts for spare blooming.
Some of the older lilacs also have alternate years where they bloom well and barely bloom at all, maybe yours had an off year last year.
Good luck with your lilacs.
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