Occasionally, whingeing reflects a more generalised miserable mood - which may be caused by hunger, fatigue or illness. Do all you can to give positive attention to non-whiny behaviour and practise calm refusals until your toddler asks properly in a reasonable tone.
If she can't talk yet, you may need to distract her with another activity when the whining starts.
Fears - the dark, the potty, insects, animals and so on.
Anxiety - especially about separation from you or being left with a new carer.
Frustration - being unable to manage or do everything she wants.
Attention - wanting you to spend more time together.
Bumps and knocks - she may be scared by how easily she can be hurt.
Your child may also be hungry, overtired or rebelling against bedtime - or she may just want to have her own way and demonstrate independence.
Whining is one behaviour guaranteed to unite parents as they strive to combat "I want, I want." It can be tempting to give in to demands for a bit of peace, but this is a mistake as it will only make the behaviour worse.
You obviously can't give in to everything your toddler wants and it always helps to explain why not.
A cuddle, a bit of praise, a few minutes spent sitting on your lap or reading a book and your child will probably be happy to go off and play again.