Aside from the possibility of a typo (so recheck everything that you entered), there are a variety of possible reasons.
The first is that the Tax Cut and Jobs Act of 2017 made a number of changes that reduced or eliminated some deductions, like for mortgage interest, state and local property and sales taxes, and moving expenses. Some taxpayers will find that they actually owe more money after the tax reform. Some taxpayers had their SALT taxes (State And Local Taxes) sharply limited. Some taxpayers were hurt by the elimination of the personal exemption. Some taxpayers were hurt by the elimination of the deduction of unreimbursed employee business expenses. And the Act increased the Child Tax Credit from $1,000 to $2,000, but that counted only for children 16 and under.
You can find a detailed list of the changes at the IRS website as https://www.irs.gov/pub/irs-pdf/p5307.pdf
Also after the Tax Cut and Jobs Act tax reform passed in 2017, the IRS updated the withholding tables to reflect the tax law changes. However, the tax reductions were not equal across the board - some taxpayers will actually owe more tax - so the IRS urged taxpayers to review their withholding all during 2018.
This means that some (many) taxpayers underwithheld compared to last year.
I don't have enough information on your return, but the tax withholding tables were another thing that changed for your return that may have had a large impact on your refund/tax due.
See https://www.irs.gov/newsroom/irs-withholding-tables-frequently-asked-questions
Note that the withholding tables are set up on the assumption that you have one job. If you have two jobs, then your total income may push you into the next tax bracket, and the W-2 withholding will have not quite enough withholding for that.
This situation has caught the attention of the national press - please see https://www.usatoday.com/story/money/2019/02/08/taxes-2019-why-my-refund-lower-2019/2816418002/
